1. Field of the Invention
The invention relates to a process for the preparation of aldehydes and ketones.
2. The Prior Art
Aldehydes and ketones are widely used in organic chemistry. For example, they are important precursors in the synthesis of heterocycles, perfumes and dyestuffs.
A variety of processes are known for the preparation of aldehydes and ketones. For example, the formyl and the acyl group are successfully introduced directly into aromatic systems via electrophilic substitution reactions, which, however, are limited by the substitution rules. Aromatic ketones can also be synthesized via organometallic reactions. A disadvantage includes the necessity of using an anhydrous reaction medium. Another disadvantage is the use of toxic chemicals such as phosphorus oxychloride, carbon monoxide, zinc cyanide, mercury organyls and cadmium organyls.
Aldehydes and ketones are frequently prepared via oxidation reactions starting from the corresponding alcohols. An overview is given in the literature reviewed in Houben-Weyl-Muller, Vol. 7/2a, p. 699 et seq. and Vol. E3, p. 265 et seq., 1983. They are most frequently carried out in nonaqueous medium and often require expensive and/or toxic reagents which are unsuitable for larger-scale syntheses because they are difficult to handle. Aromatic aldehydes and ketones can be prepared from hydroxyalkyl aromatics using stable nitroxide radicals, for example TEMPO, and an oxidant (A. E. J. de Nooy, A. C. Besemer and H. V. Bekkum, Synthesis 1996, 1153). The oxidizing agent here is an oxoammonium ion, which is formed by adding an oxidant. A disadvantage of this method is, on the one hand, the high price of reagents required. A further disadvantage is that a nonaqueous medium is required since otherwise the aldehyde formed oxidizes further to give the carboxylic acid. The formation of aromatic aldehydes from the corresponding hydroxymethyl-aromatics in the presence of the enzyme laccase and ABTS (2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) was observed by Potthast (Potthast et al., J. Molec. Catal. A: Chemical 1996, 108, and Synth. Commun. 1996, 26, 315). The high price of ABTS, however, excludes applicability of the method on an industrial scale. A further disadvantage is the low selectivity in the presence of methyl and hydroxymethyl groups.
There is therefore a demand for an inexpensive and selective process which satisfies the ever increasing demands in industry and which allows even sensitive aldehydes and ketones to be synthesized on a large scale.